Metallic descaling system

ABSTRACT

A process for descaling a metallic body, wherein the body is immersed in a bath of a molten oxidizing salt, and subsequently in an electrolyte of an aqueous solution of at least one neutral salt from the group consisting of the chloride, sulfate and nitrate of an alkali metal or ammonium.

The present invention relates to a metallic descaling process.

As anneals and other heat treatments are often carried out in oxidizingatmospheres, oxide scales are known to form on metallic surfaces. Todate, several methods of removing such scales are in commercial use.These methods do, however, necessitate the use of hot mineral acidsolutions. Typical methods are disclosed in U.S. Pat. Nos. 3,043,758 and3,260,619.

Today it is becoming increasingly clear, that there is a need for adescaling process which requires little or no acid pickling. First ofall, both the cost of using and of disposing of mineral acids hassignificantly increased. Secondly, with the continuing tightening ofpollution laws, there is a distinct possibility that the dumping ofwaste acid pickling solutions will not be permitted at some time in thefuture. Should this occur, metal producers would be forced to installvery costly acid regeneration or neutralization plants.

The present invention provides a scale removal system which eliminatesor minimizes the need for acid pickling. Applicants have found that sucha desirable result could be achieved by immersing a scaled metallic bodyin a molten oxidizing salt and then in an electrolyte on the order ofthat disclosed in U.S. Pat. No. 3,043,758. Such a process iscontradictory to the teachings of U.S. Pat. No. 3,043,758, as saidpatent does not hint upon the use of a molten oxidizing salt; and tothat of U.S. Pat. No. 3,260,619, as said patent does not hint upon theuse of an electrolyte with its molten oxidizing salt.

It is accordingly an object of the present invention to provide ametallic descaling process which eliminates or minimizes the need foracid pickling.

The present invention provides a process for descaling metallic bodies.In its broadest sense, it comprises the steps of: immersing a metallicbody in a bath of a molten oxidizing salt; and subsequently in anelectrolyte of an aqueous solution of a least one neutral salt from thegroup consisting of the chloride, sulfate and nitrate of an alkali metalor ammonium.

The molten oxidizing salt conditions the scale for subsequent removal.Of the molten oxidizing salts in use, the most widely accepted contain acompound from the group consisting of sodium nitrate and potassiumnitrate. Three typical salts are as follows:

    ______________________________________                                        A            B            C                                                   ______________________________________                                        62% NaOH     63% NaOH     62-89% NaOH                                         32% NaNO.sub.3                                                                             15% KOH      11-38% NaNO.sub.3                                    6% NaCl     12% NaNO.sub.3                                                                10% NaCl                                                         ______________________________________                                    

Immersion time in the molten salt is at least 30 seconds, and generallyin excess of 45 seconds. Times are dependent upon the size of the bodybeing descaled, as well as the type and thickness of the scale, and thetemperature of the molten salt.

Subsequent to being immersed in the molten salt, the metal being treatedis immersed in the electrolyte described hereinabove. Immersion time inthe electrolyte is at least 4 seconds, and generally at least 10. Aswith the molten oxidizing salt, immersion times are dependent upon thesize of the body being descaled, as well as the type and thickness ofthe scale, and the temperature of the electrolyte. With the electrolyte,times are also dependent upon the current density and upon the time themetal is anodic. Current densities are usually in excess of 0.1 amp persquare inch. Electrolyte temperatures are usually in the range of from120° to 200° F. Current densities, temperatures and times are all,however, dependent upon each other, as well as upon the other listedvariables. To increase the effectiveness of the electrolyte, 0.1 to 50grams per liter of a compound from the group consisting of fluorides,chlorides, perchlorates, chromates, nitrites, sulfites, nitrates andsulfates may be added. The pH of the electrolyte is maintained between1.0 to 7.0 during the electrolytic process.

Although the invention is believed to be adaptable to a number ofmetals, alloy steels presently appear to constitute the most significantembodiment thereof. For this reason, the following examples are directedto the removal of scale from stainless steel.

Samples of mill annealed Type 304 and 309 stainless steel were exposedto scale conditioning in a molten oxidizing salt of the followingchemistry:

    62% NaOH, 32% NaNO.sub.3, 6% NaCl.

and then, after rinsing, to a neutral sodium sulfate electrolyte. Thesolution concentration of sodium sulfate was between 15 and 20% byweight. The molten salt was maintained at a temperature of from 900° to950° F. whereas the electrolyte was maintained at a temperature of from150° to 170° F. Specifics as to metal gage, times, current densities andpolarities are set forth hereinbelow in Table I.

                                      TABLE I                                     __________________________________________________________________________                 SALT  ELECTROLYTE                                                             Time of                                                                             Current                                                            Gage Immersion                                                                           Density                                                                              Time of Immersion (seconds)                         Sample                                                                            Type                                                                              (inches)                                                                           (seconds)                                                                           (amp/sq.in.)                                                                         Anode                                                                             Cathode                                                                             Anode                                     __________________________________________________________________________    A   304 0.030                                                                              60    1/2     9   9     9                                        B   304 0.060                                                                              60    1/2     9   9     9                                        C   309 0.018                                                                              60    1/2    18  18    18                                        D   304 0.060                                                                              60    1/2    21  21    21                                        E   304 0.030                                                                              60    1/2    30  30    30                                        __________________________________________________________________________

Samples removed from the electrolyte were cleaned and examined forresidual scale at a magnification of 20X. The results of the examinationappear hereinbelow in Table II. Also appearing in Table II are theresults of a salt spray test. The samples were placed in a salt spraycabinet for seven days and subsequently examined for rust. Corrosionresistance of sample is somewhat impaired if scale is not removed.

                  TABLE II                                                        ______________________________________                                                                   Salt Spray                                         Sample        Scale (%)    (% Rust)                                           ______________________________________                                        A             0            0                                                  B             0            0.02                                               C             0            0                                                  D             0            0.30                                               E             0            0.05                                               ______________________________________                                    

The results appearing hereinabove in Table II clearly show that metallicsurfaces can be effectively descaled in accordance with the teachings ofthe present invention. Scale was not detected on the samples at amagnification of 20X. Moreover, salt spray results showed aninsignificant rate of attack, if any.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexamples thereof will suggest various other modifications andapplications of the same. It is accordingly desired that in construingthe breadth of the appended claims they shall not be limited to thespecific examples of the invention described herein.

We claim:
 1. A process for descaling a metallic body, which comprisesthe steps of: immersing said body in a bath of a molten oxidizing salt;maintaining said bath at a temperature above its melting point duringsaid immersion, said immersion being for a period of at least 30seconds; removing said body from said bath after scale on said body hasbeen conditioned for subsequent removal; subsequently immersing saidbody in an electrolyte of an aqueous solution of at least one neutralsalt from the group consisting of the chloride, sulfate and nitrate ofan alkali metal or ammonium; passing an electric current through saidbody and electrolyte for a period of at least 4 seconds; and removingsaid body from said electrolyte.
 2. A process according to claim 1,wherein said bath contains a compound from the group consisting ofpotassium nitrate and sodium nitrate.
 3. A process according to claim 1,wherein said electrolyte contains from 0.1 to 50 grams per liter of acompound from the group consisting of fluorides, chlorides,perchlorates, chromates, nitrites, sulfites, nitrates and sulfates.
 4. Aprocess according to claim 1, wherein the neutral salt is sodiumsulfate.
 5. A process according to claim 1, wherein said metallic bodyis an alloy steel.